Pyridine-nucleotide dependent oxidoreductase

Xanthobacter sp. strain Py2 may be grown with propene or propene oxide. On the basis of amino acid sequences, the monooxygenase that produces the epoxide was related to those that catalyzes the monooxygenation of benzene and toluene (Zhou et al. 1999). The metabolism of the epoxide is initiated by nucleophilic reaction with coenzyme M followed by dehydrogenation (Eigure 7.13a). There are alternative reactions, both of which are dependent on a pyridine nucleotide-disulfide oxidoreductase (Swaving et al. 1996 Nocek et al. 2002) ... 

Mammalian thioredoxin reductases are a family of selenium-containing pyridine nucleotide-disulfide oxidoreductases. These enzymes catalyze NADPH-dependent reduction of the redox protein thioredoxin (Trx), which contains a redox-active disulfide and dithiol group and by itself may function as an efficient cytosolic antioxidant [77]. One of the functions of Trx/ thioredoxin reductase system is the NADPH-catalyzed reduction of protein disulfide [78] ... 

The pharmaceutical and fine chemical industry might use pure hydrogenase or partially purified enzyme preparations in bioconversion applications such as regio and stereoselective hydrogenation of target compounds (van Berkel-Arts et al. 1986). Enzymes are able to catalyse such stereospecific syntheses with ease. However, the cofactors for the NAD-dependent oxidoreductases are expensive. The pyridine nucleotide-dependent hydrogenases such as those from Ralstonia eutropha and hyperthermophilic archaea (Rakhely et al. 1999) make it possible to exploit H2 as a low-cost reductant. The use of inverted micelles in hydrophobic solvents, in which H2 is soluble, has advantages in that the enzymes appear to be stabilized. 

Pyridine nucleotide-dependent flavoenzyme catalyzed reactions are known for the external monooxygenase and the disulfide oxidoreductases However, no evidence for the direct participation of the flavin semiquinone as an intermediate in catalysis has been found in these systems. In contrast, flavin semiquinones are necessary intermediates in those pyridine nucleotide-dependent enzymes in which electron transfer from the flavin involves an obligate 1-electron acceptor such as a heme or an iron-sulfur center. Examples of such enzymes include NADPH-cytochrome P4S0 reductase, NADH-cytochrome bs reductase, ferredoxin — NADP reductase, adrenodoxin reductase as well as more complex enzymes such as the mitochondrial NADH dehydrogenase and xanthine dehydrogenase. 

TrxRs are homodimeric flavoproteins [80] that catalyze the NADPH-dependent reduction of thioredoxin (Trx), a ubiquitous 12 kDa protein that is the major protein disulfide reductase in cells [81], and belongs to the pyridine nucleotide-disulfide oxidoreductase family [82]. Each monomer includes an FAD prosthetic group, a NADPH binding site and an active site containing a redox-active selenol group. Electrons are transferred from NADPH via FAD to the active-site selenol of TrxR, which then reduces the substrate Trx [83]. The crystal structure of TrxR is shown in Fig. 13 [84],... 

MDase (L-malate NAD+ oxidoreductase EC 1.1.1.37), used in homogeneous EIA, is prepared from pig heart (mitochondria). The oxidation of L-malate is generally catalyzed by two distinct pyridine nucleotide-dependent enzymes those of the malate-oxaloacetate class, which use NAD+, and those of the malate-pyruvate class (commonly known as malic enzymes), which use NADP+ (Banaszak and Bradshaw, 1975). 

Swaving, J., J.A.M. de Bont, A. Westphal, and A. de Kok. 1996. A novel type of pyridine nucleotide-disulfide oxidoreductase is essential for NAD+- and NAD-PH-dependent degradation of epoxyalkanes by Xanthobacter strain Py2. /. Bacteriol. 178 6644-6646. 

For preparative work the so far applied 2-hydroxy carboxylate oxidoreductases are pyridine nucleotide dependent (l,2a,3). The substrate specificity of the individual representatives of the various oxidoreductases is rather narrow. For thermodynamical reasons the quantitative dehydrogenation of 2-hydroxy carboxylates by pyridine nucleotide dependent redox enzymes is not so easy. But such a reaction is interesting, too. For details see Section 3.2. 

The aldehyde dehydrogenases are members of a superfamily of pyridine nucleotide [NAD(P)+]-dependant oxidoreductases that catalyze the oxidation of aldehydes to... 

Clostridium thermoaceticum contains the so-called AMAPOR (artificial-media-tor-accepting pyridine-nucleotide oxidoreductases), which are useful for electro-microbial regeneration of all four forms of pyridine nucleotides, too. An NADP(H) dependent AMAPOR from C. thermoaceticum has been purified and characterized [104]. It is able to react with rather different artificial mediators such as viologens or quinones, for example 1,4-benzoquinone, anthraquinone-2,6-disulfonate, or 2,6-dichloro-indophenol. 

Thiol enzyme. SH-enzyme an enzyme whose activity depends on the presence of a certain number of free tUol groups. T.e. are found among the hydrolases, oxidoreductases and transferases. Known T.e. are bromelain, papain, urease, various flavoenzymes, pyridine nucleotide enzymes, pyridoxal phosphate enzymes and thiolproteinases. T.e. are t ically inhibited by Sulfhydryl reagents (see). 

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